An important example of application for such a measuring system, as it is for example known from the German published patent application No. 30 25 837, is represented by measuring cables. In these cables, the components of each measuring unit as well as the components of selecting units, which each serve for the control of a respective cable portion, are miniaturised to such an extent that they can be integrated in the cable in the form of integrated circuits or hybrid circuits. Thereby, these units do not need any housing of their own.
The measuring sensors can be of quite diverse kind. For example, it is known to use thermistors as measuring sensors and to arrange the measuring units in the cable at spacings of only one meter. A cable equipped in such a manner can for example be disposed along a remote heating duct in order to monitor this. If the medium transported in the duct issues by reason of a leakage into the environment, then the temperature conditions change locally. This can be recognised through evaluation of the changed measurement values which the measuring units produce in immediate proximity of the leakage and deliver to a central control unit. In that case, the measurement values are detected with identification of the measuring unit, from which they originate. Thereby, a very accurate location-finding of an arisen leakage is possible at the same time.
Further fields of application for measuring systems of that kind result when measuring sensors for other environmental parameters are employed in place of temperature-dependent components. Thus, there are for example measuring sensors which produce an electrical signal which alters in dependence on environmental pressure or the ambient magnetic field or the ambient moisture and so forth. It is also not at all necessary that all measuring units are equipped with like measuring sensors. Thus, the temperature, the air humidity, the operational state of the illumination, the development of smoke and so forth let themselves be monitored in a plurality of rooms in a building complex which comprises many rooms and in which such a measuring system is installed. For this purpose, at least one measuring unit equipped with an appropriate measuring sensor is provided in each of the rooms concerned for one, more or each of these physical environmental parameters. All these measuring units are connected one with the other by means of the cable of the measuring system and can thus be interrogated in sequence from the central control unit.
In the case of the initially mentioned, known measuring system, the control circuit of each measuring unit preferably consists of a bistable trigger stage (flip-flop). The flip-flops of the measuring units of entire cable portions are connected one with the other into a shift register by a length of a special wire of the cable. For the performance of a measuring cycle in the cable portion concerned, a single preset binary value, for example a single logic "1" is fed into the input of the shift register. With the aid of a clock signal, which goes from the central control unit to all measuring units, this preset binary value is then advanced from measuring unit to measuring unit. The appearance of the preset binary value at an output of the flip-flop of a measuring unit leads to the closing of the associated switch arrangement. In this manner, the switches of the measuring units are then under the control of the central control unit closed in sequence for the time span between two clock pulses and then again opened. Disposed at the beginning of each cable portion is a selecting unit which can be activated with the aid of a coded address signal by way of an individual wire of the cable from the central control unit. In the activated state, the selecting unit then takes care that the preset binary value is fed into the shift register of the pertinent cable portion. During the switching-on of the system as well as also during the operation, it must be made certain that only a single measuring unit at a time generates a measurement signal. For this purpose, a resetting signal is feedable to the selecting units as well as also to the measuring units, with the aid of which resetting signal all selecting units are deactivated and all measuring unit flip-flops are reset into that state before each measuring cycle, in which their associated switch arrangement is opened. These resetting signals are transmitted from the central control unit on an individual wire of the cable to the circuits disposed in the cable. Thus, the known measuring arrangement requires a comparatively large number of cable wires and permits an access to the measuring units only portion by portion. Although the individual cable portions can be selected freely, yet the access to a certain measuring unit within a portion is possible only in the manner that all preceding measuring units are selected first.
An objective of the invention is to provide a measuring system of the initially named kind in which an optimum of possibility of access to the measuring units as well as of reliability is attainable with a smallest possible number of cable wires.